BLOOD SCIENCES
DEPARTMENT OF CLINICAL BIOCHEMISTRY
Title of Document: Glucose tolerance test: For Diagnosis of Diabetes Mellitus
Q Pulse Reference No: BS/CB/DCB/PROTOCOLS/10 Version NO: 5
Authoriser: Leila Cornes Page 2 of 6
GLUCOSE TOLERANCE TEST: FOR THE DIAGNOSIS OF DIABETES MELLITUS
Introduction
In most cases Diabetes mellitus can be diagnosed using either fasting glucose or HbA1c.
Glucose based diagnosis is possible in most cases and is the more cost effective diagnostic modality. HbA1c may be used in some patient groups, where hyperglycaemia is of more than three months’ duration and in the absence of haematological factors that could affect HbA1c formation or measurement. Diagnosis of diabetes must be based on either glucose or HbA1c but not a combination of the two. Diagnostic algorithms for glucose based diagnosis of diabetes mellitus and glucose intolerance (WHO 2006) and for HbA1c based diagnosis of diabetes mellitus (WHO 2011) are shown in Appendix A and B of this document.
Fasting glucose is recommended as the initial diagnostic test where feasible. In the absence of classical symptoms, (thirst, polyuria and unexplained weight loss) at least two diagnostic glucose results on different days are essential. If discordant results are obtained then an OGTT is recommended and either fasting (if reliable) or 2h OGTT value may be diagnostic. An OGTT is also recommended for use in the diagnosis of gestational diabetes. Note the cut-offs used locally are different to the WHO criteria applied to the non-pregnant population and are included, along with guidance on reporting by the duty biochemist in Appendix C.
Principle
Following the oral administration of a standard dose of glucose, the plasma glucose concentration normally rises but returns towards the fasting level within 2 hours. If insulin activity is reduced the plasma glucose concentration takes longer than 2 hours to return to normal.
Preparation
1) The patient should be on a normal carbohydrate diet (at least 150g per day) for at least days before the test.
2) It is desirable to postpone the test if the patient is febrile, is within 6 weeks of a myocardial infarction or major surgery or is on shortterm treatment with drugs which impair glucose tolerance, such as glucocorticoids, thiazide diuretics, beta blockers, loop diuretics, progesterone and oestrogen. (Please record any drug treatment on the request).
3) The patient should fast from 10 p.m. on the evening before the test with the exception of drinks of water and any drugs which he/she normally takes.
4) During the test the patient should be resting and should not smoke, eat or drink, except for glasses of water.
Procedure
1) At about 9 am take 2 ml venous blood into a fluoride oxalate (grey top) bottle. Note the time of the specimen on the bottle and the request form/electronic request.
2) Give the patient 75 g of glucose. This can be given in 3 different ways:
a) Lucozade
•Lucozade Energy Original is the only suitable formulation for the oral glucose tolerance test as the only sugar it contains is glucose.
All other flavour variants contain other sugars and may contain caffeine and therefore cannot be used for the oral glucose tolerance test.
•Lucozade Energy Original contains 70kcal/100ml. 410ml of Lucozade Energy Original is needed to administer the 75g glucose required for the oral glucose tolerance test, as recommended by the WHO (World Health Organisation).
• For children give 9.2 mls Lucozade per Kg up to a maximum of 394 ml (73 kcal/100 ml formulation)).
•The lucozade should be consumed over a 5 minute period. The test should be timed from the beginning of the drink.
•Some patients do not like the taste of lucozade. It is permissible for them to have a small amount of water after the lucozade drink.
b) Polycal liquid (Nutricia Clinical).
This is available on prescription. The instructions below should be followed:
•113ml Polycal liquid made up to a volume of 200ml in a beaker with water, shaken thoroughly and consumed over a 5 minute period followed by a further 100ml plain water
c) Anhydrous glucose
•75 g dissolved in 300 ml water
3) The patient needs to rest and not smoke for 2 hours until the post glucose samples are taken.
4) Take one further venous blood sample into a fluoride oxalate bottle 120 minutes after the glucose was given. Note the time of the specimen on the bottle and the request form/electronic request.
4) The test is now complete and the patient may eat. If the patient vomits during the test, stop the test.
5) Send both blood specimens with the request form (if applicable).
Interpretation (Non pregnancy)
If the above procedure is followed, venous plasma glucose is measured and the WHO criteria are as follows:
Venous plasma glucose concentrations (mmol/l)
Classification Fasting 2 Hours
Normal 6.1 and 7.8
Diabetes Mellitus ³7.0 or ³ 11.1
Impaired glucose tolerance 7.0 and 7.8 - 11.0
Impaired fasting glucose 6.1-6.9 and 7.8
NB. The above criteria apply to venous plasma. If venous whole blood or capillary blood is used, different criteria apply. Please contact laboratory.
Local diagnostic criteria in pregnancy
The local maternity guidelines use a venous fasting range of 5.5 – 7.0 mmol/L and a 2 hour range of 7.8 – 11 mmol/L to define impaired glucose tolerance. There is no separate category for impaired fasting glucose and the fasting cut-off used to define normal results is lower than the WHO guidelines. A flow chart outlining the protocol is included at the end of this document.
GTT comment
An automatic comment will be put on all GTT results:
Diabetes: Fasting glucose >7.0 mmol/L or 2hr glucose >11.0mmol/L
Impaired glucose tolerance: 2hr glucose 7.8-11.0mmol/L
Impaired fasting glucose: Fasting glucose 6.1-6.9mmol/L (>5.5mmol/L in pregnancy)
In pregnancy, all abnormal results, including impaired fasting glucose, should be referred to the antenatal clinic within 72hrs (telephone No. 0117 3235310)
Please see local maternity guidelines for referral pathways.
Related Documents
Note a patient information leaflet is downloadable from www.avondiabetes.nhs.uk
References
Definition and diagnosis of Diabetes Mellitus and intermediate hyperglycaemia. Geneva WHO 2006
Use of Glycated Haemoglobin (HbA1c) in the diagnosis of Diabetes Mellitus WHO 2011
Appendix A.
Diagnostic algorithm for glucose based diagnosis of diabetes mellitus and glucose intolerance (WHO 2006)
· Fasting glucose is recommended as the initial diagnostic test where feasible.
· In the absence of classical symptoms, (thirst, polyuria and unexplained weight loss) at least two diagnostic glucose results on different days are essential.
· If discordant results are obtained, either fasting (if reliable) or 2h OGTT value may be diagnostic, but re-testing after an interval is recommended.
· Tests should not be carried out during intercurrent illness or periods of stress as this may result in false positive results.
· Impaired Glucose Tolerance and Impaired Fasting Glycaemia carry increased risk of progression to diabetes – recommended re-assess in one year.
Appendix B.
Diagnostic algorithm for HbA1c based diagnosis of diabetes mellitus (WHO 2011)
HbA1c must not be used if hyperglycaemia has developed rapidly, for example:
· Possible Type 1 diabetes
· Symptomatic children and young adults
· Symptoms less than three months
· Acutely ill patients
· Medication that may cause rapid rise in glucose e.g. corticosteroids, antipsychotics
· Acute pancreatic damage or pancreatic surgery
HbA1c must not be used in the presence of factors affecting HbA1c formation or measurement. These include:
· Iron and vitamin B12 deficiency
· Haemolytic anaemias
· Administration of iron, vitamin B12 or erythropoietin
· Chronic liver disease
· Chronic renal failure (CKD 4 and 5)
· Alcoholism
· Rheumatoid arthritis
· Splenomagaly or splenectomy
· Haemoglobinopathies
· Drugs that may affect erythrocyte lifespan e.g. antiretrovirals, ribavarin, dapsone
HbA1c must not be used to diagnose diabetes in pregnancy
Appendix C
Screening for Diabetes in Pregnancy (local protocol)
++/+++ Glycosuria
at any routine AN visit
TIMED
GLUCOSE